proceedings: philosophical society 235 



THE PHILOSOPHICAL SOCIETY OF WASHINGTON 



The 737th meeting was held on February 28, 1914, at the Cosmos 

 Club, President Fischer in the chair; 44 persons present. The minutes 

 of the 736th meeting were read and approved. 



The evening was devoted to a symposium on allotropy. Mr. H. S. 

 Ra-Wt^on spoke on Allotropy in metals. Iron was selected as a type of 

 substances of this class and its behavior upon heating and cooling de- 

 scribed. The evidence for believing in any transformation other than 

 a — > (8 and /3 ^ 7 is too meager to be worthy of much consideration. The 

 results of the experimental work at the Bureau of Standards go to show 

 that the A2 or ;8-change takes place both on heating and coohng and 

 may be detected if the method used is refined enough. The work 

 answers the contention that the so-called |3-change is only the end of 

 the 7-change (A3) which has been depressed because of the impurities 

 which must necessarily be present. A2 does represent a phase change 

 and not merely a change in molecular equilibrium. The work has not 

 been carried far enough to offer a plausible and sufficient explanation 

 as to the real nature of the /3-phase. The paper was discussed by 

 Messrs. Silsbee and Humphreys. 



Mr. C. N. Fenner presented a paper on Allotropy as illustrated by 

 the forms of silica. Silica is remarkable for the number of allotropic 

 modifications which it shows and the way in wh'ch the various forms 

 pass into each other throws some light on the molecular processes con- 

 cerned in transformations. The relations of the different phases to 

 each other were represented on a diagram in which the coordinates are 

 temperature and vapor pressure and attention directed to the peculiar 

 behavior of cristobalite at the a — > /3 inversion point. Smith's theory 

 that frequently two or several kinds of molecules are present in the same 

 crystal structure offers an explanation. His theory that every sub- 

 stance having an inversion point must consist of several kinds of mole- 

 cules is not so plausible. A crystal is a complex structure in which the 

 molecules are held together in a definite pattern. Under conditions of 

 stability there is an equilibrium of forces, but at an inversion point some 

 system of forces becomes unstable, so that molecules may either arrange 

 themselves differently in the crystal or change may involve the struc- 

 ture of the molecules themselves. This may possibly explain the two radi- 

 cally unlike types of inversion shown by different forms of silica. Paper 

 was discussed by Mr. White. 



Mr. R. B. SosiviAN spoke on Allotropy from the physico-chemical stand- 

 point. Allotropy or polymorphism is only a special case of the general 

 phenomenon of isomerism, which includes all cases of substances having 

 the same chemical composition by weight but differing in properties. A 

 consideration of the various kinds of isomerism known shows that sharp 

 lines can not be dra^\Ti between the different types. The phenomenon 

 seems in all cases dependent upon differences in molecular weight or 

 molecular structure. Recent theories of the thermodynamic side of 

 the subject were discussed. Mr. C. A. Briggs made some remarks 

 upon the paper. J. k. Fleming, Secretary. 



